X-ray tubes, gas-filled X-ray detectors and various other applications require window materials applicable to sealing an opening in a gastight manner, while still letting X-rays of at least some desired wavelength range pass through the window with as little attenuation as possible. Another requirement for the window material is its ability to stand a certain amount of mechanical stress, because the pressure difference between the different sides of the window may be considerable.
In this description we use the terms “film” and “foil” to mean a thin material layer of uniform thickness, and the term “membrane” to mean generally a structure that is relatively thin, i.e. has a very small overall dimension in one direction compared to its dimensions in the other, perpendicular dimensions. A membrane may consist of several materials and may have significant local variations in its thickness, and may exhibit structural topology, such as reinforcement ridges. Additionally we use the term “layer” to mean a thin amount of material, which does not necessarily need to be continuous or even but which consists of essentially a single constituent. A “mesh” is a special case of a layer, in which intentional discontinuities exist usually in the form of a regular matrix of openings.
Films and membranes for radiation windows can be manufactured in various ways. One commonly used material is beryllium, from which high-quality films as thin as 8 micrometers can be manufactured by rolling. On a base membrane various additional layers can be produced using thin film manufacturing techniques such as sputtering or chemical vapor deposition. A drawback of known membranes for radiation windows is the possible appearance of pinholes, which are microscopic discontinuities in an otherwise continuous material layer. Pinholes may allow gas to leak through, which causes contamination of gas-filled enclosures with unwanted gaseous substances as well as degradation of intended overpressure or vacuum environments.